Terry Pratchett and Stephen Briggs wrote:Its tiny orbiting sunlet, with prominences no bigger than croquet hoops, maintains a fixed elliptical orbit, while the Disc revolves beneath it, taking about 800 days to complete the full circle. The little moon shines by it's own light, owing to cramped and rather inefficient astronomical arrangements.
The Colour of Magic wrote:The shape and cosmology of the disc system are perhaps worthy of note at this point. There are, of course, two major directions on the disc: Hubward and Rimward. But since the disc itself revolves at the rate of once every eight hundred days (in order to distribute the weight fairly upon its supportive pachyderms, according to Reforgule of Krull) there are also two lesser directions, which are Turnwise and Widdershins. Since the disc’s tiny orbiting sunlet maintains a fixed orbit while the majestic disc turns slowly beneath it, it will be readily deduced that a disc year consists of not four but eight seasons. The summers are those times when the sun rises or sets at the nearest point on the Rim, the winters those occasions when it rises or sets at a point around ninety degrees along the circumference. Thus, in the lands around the Circle Sea, the year begins on Hogs’ Watch Night, progresses through a Spring Prime to its first midsummer (Small Gods’ Eve) which is followed by Autumn Prime and, straddling the half-year point of Crueltide, Winter Secundus (also known as the Spindlewinter, since at this time the sun rises in the direction of spin). Then comes Secundus Spring with Summer Two on its heels, the three quarter mark of the year being the night of Alls Fallow—the one night of the year, according to legend, when witches and warlocks stay in bed. Then drifting leaves and frosty nights drag on towards Backspindlewinter and a new Hogs’ Watch Night nestling like a frozen jewel at its heart.
Since the Hub is never closely warmed by the weak sun the lands there are locked in permafrost. The Rim, on the other hand, is a region of sunny islands and balmy days. There are, of course, eight days in a disc week and eight colours in its light spectrum.
The Colour of Magic wrote:The Discworld offers sights far more impressive than those found in universes built by Creators with less imagination but more mechanical aptitude. Although the disc’s sun is but an orbiting moonlet, its prominences hardly bigger than croquet hoops, this slight drawback must be set against the tremendous sight of Great A’Tuin the Turtle, upon Whose ancient and meteor-riddled shell the disc ultimately rests. Sometimes, in His slow journey across the shores of infinity, He moves His countrysized head to snap at a passing comet.
Wyrd Sisters wrote:Through the fathomless deeps of space swims the star turtle Great A'Tuin, bearing on its back the four giant elephants who carry on their shoulders the mass of the Discworld. A tiny sun and moon spin around them, on a complicated orbit to induce seasons, so probably nowhere else in the multiverse is it sometimes necessary for an elephant to cock a leg to allow the sun to go past.
Tonyblack wrote:There are a few things that were abandoned I think. Like the 800 day year - that would mean only one Hogswatch every 800 days.